1. Field of the Invention
Aspects of the present invention generally relate to a fixing apparatus mounted on an image forming apparatus using an electrophotographic method such as a printer, a copier, or a fax machine for fixing a toner image formed on a recording material onto the recording material.
2. Description of the Related Art
In recent years, fixing apparatuses employing a method called film heating method has been widely used. A fixing apparatus employing the film heating method includes a film, a ceramic heater contacting the inner face of the film, and a pressure roller. The pressure roller is a pressure-applying rotating member which forms a nip portion together with the ceramic heater via the film. At the nip portion, a toner image is fixed onto a recording material by an application of heat while the recording material with the toner image passes through the portion.
The above-described fixing apparatus is advantageous in that if a film with a small heat capacity is used, the time required for the film to reach the fixing temperature (i.e., warm-up time) can be reduced and, further, power consumption can be reduced.
As a pressure roller for the above-described fixing apparatus employing the film heating method, a pressure roller with a so-called reverse crown shape is widely used. The outer diameter of such a pressure roller is the smallest at the center portion and gradually increases toward the end portions in the longitudinal direction.
If this pressure roller is used, when a recording material passes through the nip portion, the recording material is conveyed relatively faster at the end portions in the longitudinal direction compared to the center portion. Accordingly, the occurrence of warp and crease of the recording material at the nip portion can be reduced.
However, even if a pressure roller with the reverse crown shape is used, if the amount of reverse crown is reduced, the effect of preventing the crease of the recording material will be reduced.
For example, since the heat dissipation at the end portions of the pressure roller in the longitudinal direction is greater than the heat dissipation at the center portion of the pressure roller, the temperature at the end portions of the pressure roller in the longitudinal direction may be lower than the temperature at the center portion. In this case, since the thermal expansion at the center portion of the pressure roller in the longitudinal direction will be greater than the end portions of the pressure roller, the amount of reverse crown is reduced compared to when the pressure roller is cooled.
Further, there may be a pressure roller having the reverse crown shape covered with a straight tube outside the rubber layer having the reverse crown shape. The tube serves as a release layer. A pressure roller 22 illustrated in FIG. 9 is a reverse crown shape pressure roller having a rubber layer 27 which also has a reverse crown shape and is covered with a tube 28 as a straight shape release layer.
With this pressure roller, a tension F2 which the rubber layer receives from the tube at the end portions is greater than a tension F1 at the center portion of the pressure roller in the longitudinal direction. Thus, if the whole pressure roller 22 is heated, since the center portion tends to expand more due to thermal expansion of the rubber, the amount of reverse crown of the pressure roller is reduced.
Japanese Patent Application Laid-Open No. 2005-195856 discusses a pressure roller having the amount of reverse crown set with a sufficient margin in a predetermined temperature state so that the predetermined amount of reverse crown can be maintained.
However, when the amount of reverse crown of a pressure roller is set as discussed in Japanese Patent Application Laid-Open No. 2005-195856, if the pressure roller is cooled as is the case when the printing is started, the conveying speed of the recording material at the end portions will be faster compared to the center portion of the pressure roller in the longitudinal direction, and the recording material may be warped. Then, according to this warp, the recording material may be deformed and greatly spring up. Accordingly, the toner image on the recording material contacts, for example, a guide and a defective image may be generated.